opt_clean: Better memory handling.

Previously, `$memwr` and `$meminit` cells were always preserved (along
with the memory itself).  With this change, they are instead part of the
main cell mark-and-sweep pass: a memory (and its `$meminit` and `$memwr`
cells) is only preserved iff any associated `$memrd` cell needs to be
preserved.

passes/opt/opt_clean.cc

@@ -55,7 +55,7 @@ struct keep_cache_t
 		if (!module->get_bool_attribute(ID::keep)) {
 		    bool found_keep = false;
 		    for (auto cell : module->cells())
-			if (query(cell, true /* ignore_specify_mem */)) {
+			if (query(cell, true /* ignore_specify */)) {
 			    found_keep = true;
 			    break;
 			}
@@ -70,12 +70,12 @@ struct keep_cache_t
 		return cache[module];
 	}
 
-	bool query(Cell *cell, bool ignore_specify_mem = false)
+	bool query(Cell *cell, bool ignore_specify = false)
 	{
 		if (cell->type.in(ID($assert), ID($assume), ID($live), ID($fair), ID($cover)))
 			return true;
 
-		if (!ignore_specify_mem && cell->type.in(ID($memwr), ID($meminit), ID($specify2), ID($specify3), ID($specrule)))
+		if (!ignore_specify && cell->type.in(ID($specify2), ID($specify3), ID($specrule)))
 			return true;
 
 		if (cell->has_keep_attr())
@@ -95,6 +95,8 @@ int count_rm_cells, count_rm_wires;
 void rmunused_module_cells(Module *module, bool verbose)
 {
 	SigMap sigmap(module);
+	dict<IdString, pool<Cell*>> mem2cells;
+	pool<IdString> mem_unused;
 	pool<Cell*> queue, unused;
 	pool<SigBit> used_raw_bits;
 	dict<SigBit, pool<Cell*>> wire2driver;
@@ -108,6 +110,17 @@ void rmunused_module_cells(Module *module, bool verbose)
 		}
 	}
 
+	for (auto &it : module->memories) {
+		mem_unused.insert(it.first);
+	}
+
+	for (Cell *cell : module->cells()) {
+		if (cell->type.in(ID($memwr), ID($meminit))) {
+			IdString mem_id = cell->getParam(ID::MEMID).decode_string();
+			mem2cells[mem_id].insert(cell);
+		}
+	}
+
 	for (auto &it : module->cells_) {
 		Cell *cell = it.second;
 		for (auto &it2 : cell->connections()) {
@@ -145,17 +158,33 @@ void rmunused_module_cells(Module *module, bool verbose)
 	while (!queue.empty())
 	{
 		pool<SigBit> bits;
-		for (auto cell : queue)
-		for (auto &it : cell->connections())
-			if (!ct_all.cell_known(cell->type) || ct_all.cell_input(cell->type, it.first))
-				for (auto bit : sigmap(it.second))
-					bits.insert(bit);
+		pool<IdString> mems;
+		for (auto cell : queue) {
+			for (auto &it : cell->connections())
+				if (!ct_all.cell_known(cell->type) || ct_all.cell_input(cell->type, it.first))
+					for (auto bit : sigmap(it.second))
+						bits.insert(bit);
+
+			if (cell->type == ID($memrd)) {
+				IdString mem_id = cell->getParam(ID::MEMID).decode_string();
+				if (mem_unused.count(mem_id)) {
+					mem_unused.erase(mem_id);
+					mems.insert(mem_id);
+				}
+			}
+		}
 
 		queue.clear();
+
 		for (auto bit : bits)
 		for (auto c : wire2driver[bit])
 			if (unused.count(c))
 				queue.insert(c), unused.erase(c);
+
+		for (auto mem : mems)
+		for (auto c : mem2cells[mem])
+			if (unused.count(c))
+				queue.insert(c), unused.erase(c);
 	}
 
 	unused.sort(RTLIL::sort_by_name_id<RTLIL::Cell>());
@@ -168,6 +197,14 @@ void rmunused_module_cells(Module *module, bool verbose)
 		count_rm_cells++;
 	}
 
+	for (auto it : mem_unused)
+	{
+		if (verbose)
+			log_debug("  removing unused memory `%s'.\n", it.c_str());
+		delete module->memories.at(it);
+		module->memories.erase(it);
+	}
+
 	for (auto &it : module->cells_) {
 		Cell *cell = it.second;
 		for (auto &it2 : cell->connections()) {

tests/opt/opt_clean_mem.ys

@@ -0,0 +1,49 @@
+read_verilog <<EOT
+module top(...);
+
+input [7:0] wa;
+input [7:0] ra1;
+input [7:0] ra2;
+input [7:0] wd;
+input clk;
+wire [7:0] rd1;
+wire [7:0] rd2;
+
+reg [7:0] mem[0:7];
+
+always @(posedge clk)
+	mem[wa] <= wd;
+assign rd1 = mem[ra1];
+assign rd2 = mem[ra2];
+
+initial mem[8'h12] = 8'h34;
+
+endmodule
+EOT
+
+proc
+memory_dff
+
+select -assert-count 2 t:$memrd
+select -assert-count 1 t:$memwr
+select -assert-count 1 t:$meminit
+design -save orig
+
+opt_clean
+select -assert-none t:$memrd
+select -assert-none t:$memwr
+select -assert-none t:$meminit
+
+design -load orig
+expose top/rd1
+opt_clean
+select -assert-count 1 t:$memrd
+select -assert-count 1 t:$memwr
+select -assert-count 1 t:$meminit
+
+design -load orig
+expose top/rd1 top/rd2
+opt_clean
+select -assert-count 2 t:$memrd
+select -assert-count 1 t:$memwr
+select -assert-count 1 t:$meminit